1. Field of the Invention
This invention relates to a method and apparatus for measuring the energy of a signal pulse, by integrating the signal intensity of the pulse waveform of the input signal pulse. This invention is widely used not only in measurements of the energy of radiation and in dosimetry, but in measurement of radiation detection positions, radiation images and other areas, and in particular is applied in gamma cameras used in nuclear medical diagnostics, in SPECT (Single Photon Emission Computed Tomography) systems, and in PET (Positron Emission Tomography) systems.
2. Description of the Related Art
When performing measurements of γ rays, charged particle beams and other radiation (energy beams), a scintillation detector using a scintillator, or some other radiation detector is used. The detection signal output from the radiation detector is subjected to prescribed signal processing or to other processing to obtain the required information.
For example, using a scintillation detector, radiation incident on the scintillator is detected through the scintillation light pulses occurring in the scintillator. Optical signal pulses resulting from this scintillation light are converted into electrical signal pulses by a photomultiplier tube or other photodetector. That is, when the scintillation light is incident on the photoelectric surface of a photomultiplier tube, a plurality of photoelectrons are generated from the photoelectric surface, in proportion to the light intensity; after these photoelectrons are collected by a first dynode, the electrical signal is amplified by successive dynodes in sequence, and output as a pulse signal (electrical current signal).
In general, the scintillation light of a scintillator used in a radiation detector has a pulse waveform, with the signal intensity being attenuated exponentially, for example. The total number of photoelectrons collected by the first dynode corresponds to the energy of the radiation absorbed by the scintillator. Hence in order to measure the energy of the radiation, the output signal from a photomultiplier tube must be integrated over an appropriate time interval. In general, the total number of photoelectrons collected by the first dynode as a result of one signal pulse is not sufficiently large, and so it is preferable that the above integration time be set so as to integrate most of the scintillation light. If the integration time is short, the number of photoelectrons collected is decreased, and so the energy resolution is degraded due to statistical fluctuations.
If measurements are performed in a state in which the number of detections (count rate) of radiation per unit time by the radiation detector is high, then the probability is increased that the pulse interval between signal pulses will be approximately the same as, or shorter than, the pulse width of individual pulses, so that so-called “pileups” in which two or more signal pulses overlap temporally occur. At such times, if the signal intensity (current signal) of a signal pulse the energy of which is to be measured is integrated, the signal intensity of another signal pulse with which the signal pulse has piled-up is simultaneously integrated, and so there arises the problem that the energy of the signal pulse being measured cannot be accurately measured.
When a pulse waveform is represented by a single exponential function, a comparatively simple method which is conventionally used to reduce the error due to pileups involves shortening the time width of pulses using the delay line clipping method, and setting the integration time to be approximately equal to the pulse time width. In this case, the shorter the pulse time width is made, the more the integration time can be shortened, so that the probability of occurrence of pileups is decreased at high count rates, and the count rate characteristic can be improved. On the other hand, the number of photoelectrons collected at the first dynode of the photomultiplier tube for each signal pulse is reduced, and so there is the drawback that even at low count rates at which pileups do not occur, the energy resolution is lowered.
As prior art which improves on this, in the method of Tanaka et al (reference 1: Nucl. Instr. Meth. Vol. 158, pp. 459-466, 1979), the pulse time width is shortened by a delay line clipping method like that above, but by controlling the integration time through the occurrence of the succeeding pulse, so that the integration time is sufficiently long within the range in which the succeeding pulse does not occur, a lowering in energy resolution at low count rates is avoided.
In the method of Kolodziejczyk (reference 2: U.S. Pat. No. 5,430,406), by adding the pulse signal (current signal) and the integration signal obtained by time integration of this, with appropriate weighting, an addition signal is generated which is constant in time and the amplitude of which is proportional to the energy; measurements are performed by sampling the amplitude of this addition signal. The addition signal of the signal pulse for measurement is sampled immediately before the arrival of the succeeding signal pulse; by measuring this value, the effect of pileups of succeeding signal pulses can be eliminated, but the effect of pileups of preceding signal pulses cannot be eliminated.
Still another method is that of Wong (reference 3: International Patent WO98/50802). Similarly to Kolodziejczyk's method described above, this method employs a technique in which an addition signal obtained from the current signal and an integration signal is measured, but is improved so as to correct for the effect of all signal pulses arriving before the signal pulse being measured.